Allow site-search to work with --restrict-instant-to-search.

Allow site-search to work with --restrict-instant-to-search.

With --restrict-instant-to-search (and Instant turned on, of course), type
[google] into the omnibox, and select "Search Google for ..." from the drop
down. Continue typing. Without this patch, no preview results appear. With this
patch, preview results appear. I.e., there's at least one SEARCH_OTHER_ENGINE
that provides Instant functionality, viz., Google, so it should be a part of
the accepted autocomplete match types for the "search only" mode.

BUG=none
TEST=Tested the scenario in the description.

[sreeram, 2011-05-06 22:14:18]

Please review.

[sky, 2011-05-06 22:22:22]

LGTM